1. Field of the Invention
This invention relates to new diisocyanates which contain urethane groups and are soluble in hydrocarbon solvents, a process for their preparation and their use as binders or binder component in one-component or two-component polyurethane lacquers.
2. Description of the Prior Art
Polyisocyanates which contain urethane groups and are based on aromatic or aliphatic diisocyanates and low molecular weight polyhydric alcohols constitute an important group of polyisocyanates used in lacquer technology (see Kunststoff-Handbuch by G. W. Becker and D. Braun, Carl Hanser Verlag, Munich (1983), Volume 7, "Polyurethane", 2nd Edition, in particular Chapter 3.3.2.1).
The advantage of such polyisocyanates containing urethane groups compared with the monomeric diisocyanates on which they are based is that they are not volatile and are therefore easier and safer to handle. They are generally prepared by reacting low molecular weight polyols such as glycerol, trimethylolpropane, hexanetriol and/or pentaerythritol, optionally in admixture with diols, with excess quantities of simple diisocyanates such as 2,4-diisocyanatotoluene, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, abbreviated IPDI) or 1,6-diisocyanatohexane. The reaction is accompanied by urethane formation and, subsequently, the excess, unreacted starting diisocyanate is removed by distillation. The polyisocyanates containing urethane groups are generally resinous substances used as solutions in solvents. The solvents are generally polar solvents such as esters of acetic acid, e.g. ethyl acetate or ethyl glycol acetate, optionally in admixture with solvents which are only slightly polar such as hydrocarbons. It is generally not possible to use hydrocarbons alone as solvents since the solutions obtained become very cloudy when left to stand or when diluted to the working concentration and, therefore, become unusable. Many polyester polyols and alkyd resins which may be used as potential co-reactants for lacquer polyisocyanates in two-component polyurethane lacquers are in practice also used as solutions in hydrocarbon solvents, but due to the poor solubility or incompatibility of the above-mentioned state of the art lacquer polyisocyanates with hydrocarbon solvents, the solutions frequently become cloudy or precipitates form when the polyol components dissolved in hydrocarbon solvents are mixed with solutions of the lacquer polyisocyanates, even if the polyisocyanate component was previously dissolved in a mixture of polar and apolar solvents. Such cloudiness and formation of precipitates reduce the gloss and damage the surface of coatings produced from the lacquers.
The consequent necessity to use polar solvents, in particular ester-based polar solvents, in the two-component polyurethane lacquers known in the art has the further disadvantage that polar solvents such as the esters of acetic acid which are most commonly used, are retained for a long time in small quantities in the hardened lacquer film. If lacquers are used externally and therefore exposed to the weather, these esters undergo hydrolysis, and the acetic acid formed has two very disadvantageous effects, i.e., it catalyzes the degradation of the lacquer film and in lacquers applied to metal, it reduces the corrosion protection provided by the lacquer. Both these factors reduce the life of the lacquer coat.
It was therefore an object of the present invention to provide lacquer polyisocyanates which form clear solutions in aromatic hydrocarbons which are only slightly polar, and to provide solutions which could be further diluted to the required working concentration by means of apolar solvents such as aromatic or aliphatic hydrocarbons. It is also an object to provide diisocyanates which have a lower inherent viscosity than related prior art polyisocyanates and, thus, may be used in the known applications for these polyisocyanates at higher solids contents.
These problems were able to be solved by the diisocyanates containing urethane groups according to the invention described below and the process according to the invention for the preparation of these diisocyanates which is also described below.